For a variety of load-operating purposes it is advantageous to couple a prime mover, such as an internal combustion engine, with a load via a hydrostatic transmission, with or without a mechanical transmission. In a typical hydrostatic transmission, the internal combustion engine drives a so-called hydrostatic pump which generally has a drum rotatable about an axis and provided with a plurality of angularly equispaced pistons which cooperate with a control plate inclined to the axis so that each piston is reciprocated in a respective cylinder upon rotation of the drum. Alternatively, the hydrostatic pump can have a stationary drum and the excursion of the pistons can be induced by the rotation of an inclined plate which is frequently referred to as a swash plate.
In axial piston pumps the drum abuts or communicates with a valve plate or control plate having kidney-shaped or arcuate openings communicating with the high-pressure and low pressure ports and any relief, pressure-control or other valves which may be required. The other side of the hydrostatic transmission comprises a hydrostatic motor which likewise has a drum formed with axial pistons and rotatable relative to a valve plate while its axial pistons bear upon an inclined plate connected to a shaft or reacting against an inclined plate while the drum is connected to the shaft. The two ports of the valve plate of the motor are connected to the two ports of the pump and, upon rotation of the pump and depending upon the inclination of the plate thereof, the motor is driven in one or another sense at a variable speed.
Reference is made, in this connection, to pages 397 and 398 of Servomechanism Practice, McGraw-Hill Book Company, New York, 1960 wherein a pump and motor of the aforedescribed type are illustrated.
The hydrostatic motor, therefore, comprises three basic elements, namely, the valve plate through which fluid communication is made to the cylinder drum which cooperates with this plate and has the angularly spaced cylinder bores and pistons, and the output shaft which is connected to the inclined plate and/or drum against which the pistons react.
In hydrostatic-mechanical drive systems, the output shaft of the motor can be connected to the load via a mechanical transmission, e.g. planetary gearing, and reference is made to the aforementioned copending application which shows a typical application of a hydrostatic-mechanical transmission in which the pump is driven by a prime mover and is hydraulically connected to a fixed-displacement hydrostatic motor in turn connected to a cement mixer drum by a mechanical transmission. Other typical applications of hydrostatic motors in construction machinery and vehicles are the drive of a crane turntable or cable drum, individual drives for the vehicle wheels, and control of booms of various types.
For all these application it is desirable to monitor the operation of the load, generally at a location remote therefrom, especially since the response of a hydrostatic motor to a change in conditions at the prime mover or a pump adjustment, may not always provide the same response at the hydrostatic motor. Such monitoring is advantageous for control of driving conditions when the vehicle is traveling or for sensitive functioning of the load and to ensure maximum operating efficiency.
A variety of electrical and electronic control and display arrangements have been provided heretofore for operating conditions of a load and their importance has led to widespread application of electronic techniques in monitoring.
For example, in construction and other machinery operated by a single person for reasons of economy, the operator cabin is centrally located, often remote from a particular load although the latter is controlled by him. The operator may not even be able to see the load and frequently is unable to visually ascertain a parameter of its operation, for example, its speed. In such cases, the operating speed of the load, although an important and frequently vital item of information, may not be available in the absence of electronic monitoring systems. Since such operating parameters may not be available, obviously the response to an operator control may not be easily monitored.
When it has been desirable to electrically or electronically monitor the rotary speed of the load, a tachometer or the like could generally be coupled to a rotating portion to provide the desired output.
However, in the case of hydrostatic-mechanical drives of the aforementioned type this has frequently been impossible or impractical.
Because such drives are often used where space is at a premium, the hydrostatic motor and any directly adjoining speed-reducing mechanical gearing are generally provided in a compact housing directly adjoining the load. For practical reasons it is difficult if not impossible to monitor the speed of the mechanical transmission elements such as gearing and the like and, for the most part, electronic control of such systems has not been effected although the advantages of modern electronic monitoring systems, for example, microprocessor controls, in conjunction with accurate input information as to speed, has been recognized heretofor.
The simple attachment of an ordinary tachometer to the shaft of the drive, i.e. a part with a relatively low peripheral speed, has proved to be inadequate. The output frequencies of such tachometers was too low to enable it to be used in conjunction with microprocessors and like circuitry.